It's 18

the sum of the first six numbers is :4 .6 =24

the sum of the seven numbers is :5 *7 =35

the seventh number is :35 -24=11

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Total of 6 numbers are:
  4 x 6 = 24.
Total of 7 of them are:
5 x 7 = 35.
7th number is:
35-24 = 11.
Answer: 11

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I call the number of students didn't take part in Mathemas Competition in class A & B are a & b respectively

(with condition: a & b \(\in\)N*)

,From the theme, we have: \(\frac{1}{3}b+a=\frac{1}{5}a+b\)(Because rhe number of students in 2 class is the same)

\(\Leftrightarrow\frac{b}{3}-b+a-\frac{a}{5}=0\Leftrightarrow\left(-\frac{2b}{3}\right)+\frac{4a}{5}=0\)

\(\Leftrightarrow\frac{4a}{5}-\frac{2b}{3}=0\Leftrightarrow\frac{12a-10b}{15}=0\Leftrightarrow12a=10b\)

\(\Rightarrow\frac{a}{b}=\frac{10}{12}=\frac{5}{6}\) 

So the ratio of the number of student didn't take part of Class A & B is \(\frac{5}{6}.\)

Tham khảo
length = 19 feets and breadth = 18 feets
Given, area of the dining room = 342 square feet.
We know that, area of rectangle = length x breadth
Implies, the length and breadth are the factors of 342
Prime factorisation of 342 = 2 x 3 x 3 x 19
Given that the approximate length of one side is a prime number less than 25.
The nearest prime number less than 25 and a factor of 342 is 19.
Therefore, the approximate length of the room = 19 feet
Hence, breadth =\(\dfrac{area}{length}\)=\(\dfrac{342}{19}\)=18 feet

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